POLLINATION 



419 



foot slips into a similar slit the pollen masses are drawn into the 

 stigmatic cavity, and in part or wholly torn off by the struggle 

 of the insect. Occasionally an insect is not strong enough to 

 free itself from these traps and perishes, suspended by one or 

 several feet. This is the only possible method of pollination in 

 milkweeds. 



852. The flowers of the Yucca are pollinated by the Pronuba 

 moth. The moth deliberately collects the pollen with the fore 

 feet, then goes to the pistil of the same flower, pierces it with 

 her ovipositor and deposits an egg. She then goes to the fun- 



FiG. 256. — A wasp, Sphex ichneumonea, with a number of the milk weed pollen 



masses attached to its feet. 



nel-like stigma and deposits the pollen. This operation is re- 

 peated in the same and neighboring flowers until her eggs are all 

 deposited. The carrying of pollen to the stigma is an indirect 

 method of providing food for the young, since it causes the 

 ovules to develop, and these are devoured by the larvae of the 

 moth after hatching. When ready to pupate the larvae escape 

 from the ovary by a hole which they bore through its wall. 

 Not all the developing seeds are devoured. The plant sacri- 

 fices a part of the seeds as a reward for the services of 

 pollination. 



853, It is not only the plants that are affected by the adapta- 



